Children with Attention Deficit Hyperactivity Disorder (ADHD) have more trouble identifying odors from scratch-and-sniff patches than their peers.

The findings in a University of Melbourne study published in the Journal of Clinical Psychiatry fit with recent research showing that war veterans with post-traumatic stress disorder also experience a reduced capacity to identify smells (AS, June 2008, p.7). Mental illnesses such as schizophrenia also have similar effects [AS, Jan/Feb 2004, p.48).

Prior research had already established an association between ADHD and smell recognition in adults, but this is the first test among children with the sample group aged 6-16.

"The sense of smell is increasingly becoming a useful way to detect a vulnerability to a neuropsychiatric disorder," says Prof Warrick Brewer of the ORYGEN Youth Health Research Centre. Brewer adds that smell recognition could be used as a screening test but would not substitute for "an interview with a well-qualified pediatrician" for definitive diagnosis. However, he says that smell tests may indicate a primary brain development cause for mental health issues and antisocial behavior.

The researchers tested both nostrils and found a much clearer effect in the right nostril. Brewer says this is because "smell, unlike the other senses, is highly lateralised, and being more a primal sense it is likely linked to reflexive action where there is an advantage if processing is performed directly rather than swapping sides of the brain".

Brewer says it is unlikely that the problem is a more general memory deficit in children with ADHD. Studies of adults have established that recognition of smells can be impaired in mental illness when general intelligence is not, and the test was run in a multiple-choice fashion so "not a lot of load was put on working or long-term memory".

The ADHD group had a mean intelligence close to 100, and included some highly intelligent children.